igneous
Porphyritic volcanic rock, likely rhyolite or andesite
Porphyritic rhyolite/andesite; phenocrysts in a fine-grained volcanic groundmass
AI-generated identification · may be incorrectLight brown-gray matrix with abundant pale pink-white and possibly orange feldspar phenocrysts; likely Mohs 6–7, dull to slightly vitreous luster, massive texture, and no obvious cleavage at hand-specimen scale. Exact identification requires thin-section or chemical testing.
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Physical properties
Light brown-gray matrix with abundant pale pink-white and possibly orange feldspar phenocrysts; likely Mohs 6–7, dull to slightly vitreous luster, massive texture, and no obvious cleavage at hand-specimen scale. Exact identification requires thin-section or chemical testing.
Formation & geological history
Formed when silica-rich to intermediate lava cooled rapidly at or near Earth’s surface, preserving larger crystals formed earlier underground; likely Cenozoic to older, depending on locality.
Uses & applications
Used mainly as a decorative or educational specimen and occasionally as crushed aggregate or building stone; generally no jewelry value unless attractively polished.
Geological facts
The large crystals are phenocrysts, while the fine matrix is the volcanic groundmass—a texture indicating two-stage cooling.
Field identification & locations
Look for abundant embedded feldspar crystals, a dense fine matrix, and possible quartz; distinguish rhyolite from andesite by overall silica content, color, and mineral proportions. Common in volcanic belts worldwide.
